<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script type="text/javascript">
        window.onload =function () {
            let elementById = document.getElementById("select");
            elementById.onchange=function () {
                alert("has been changed");
            }
        }
    </script>
</head>
<body>
    <select id="select">
        <option>one</option>
        <option>two</option>
        <option>three</option>
    </select>
</body>
</html>